Not applicable.
The present invention relates to miniature acoustic transducers, and in particular to a miniature acoustic transducer having a diaphragm with a planar receiving surface that remains substantially planar while deflecting pressure waves.
The batch fabrication of acoustic transducers using similar processes as those known from the integrated circuit technology offers interesting features with regard to production cost, repeatability and size reduction. Furthermore, the technology offers the unique possibility of constructing a single transducer having a wide bandwidth of operation with a uniform high sensitivity. This provides for a transducer that, with little or no modification, can be used in such diverse applications as communications, audio, and ultrasonic ranging, imaging and motion detection systems.
The key to achieve wide bandwidth and high sensitivity lies in creating a structure having a small and extremely sensitive diaphragm. Designs have previously been suggested U.S. Pat. No. 5,146,435 to Bernstein, and in U.S. Pat. No. 5,452,268 to Bernstein. In these structures the diaphragm is suspended on a number of very flexible movable springs. However, the implementation of the springs leads to an inherent problem of controlling the acoustic leakage in the structure, which in turn affects the low frequency roll-off of the transducer. Another approach is to suspend the diaphragm in a single point, which also provides an extremely sensitive structure. See U.S. Pat. No. 5,490,220 to Loeppert. Unfortunately, in this case the properties of the diaphragm material become critical, especially the intrinsic stress gradient which causes a free film to curl. Eventually, this leads to a similar problem for this structure concerning the reproducibility of the low frequency roll-off of the transducer.
The present invention results from a realization that a diaphragm has the highest mechanical sensitivity if it is free to move in its own plane. Furthermore, if the diaphragm is resting on a support ring attached to the perforated member, a tight acoustical seal can be achieved leading to a well controlled low frequency roll-off of the transducer. Additionally, if a suspension method is chosen such that the suspension only allows the diaphragm to move in its own plane and does not take part in the deflection of the diaphragm to an incident sound pressure wave, complete decoupling from the perforated member can be achieved which reduces the sensitivity to external stresses on the transducer.
In one embodiment the present invention features an acoustic transducer consisting of a perforated member and a movable diaphragm spaced from the perforated member. The spacing is maintained by a support ring attached to the perforated member upon which the diaphragm rests. There are means for suspending the diaphragm such that the diaphragm is free to move in its own plane, thereby maximizing the mechanical sensitivity of the diagram. The suspension is achieved by restraining the diaphragm laterally between the support ring and the substrate attached to the perforated member. There are means for applying an electrical field between the perforated member and the diaphragm. There are also means for detecting the change of electrical capacitance between the perforated member and the diaphragm as the diaphragm deflects due to an incident acoustic sound pressure wave.
The thickness and size of the diaphragm are chosen such that the resonance frequency of the diaphragm is larger than the maximum acoustical operating frequency. Similarly, the dimensions of the perforated member are chosen such that the resonance frequency is larger than the maximum acoustical operating frequency. The perimeter at which the perforated member is attached to the substrate can optionally be shaped to minimize the curvature of the perforated member due to intrinsic stress in said perforated member. The suspension means of the diaphragm are made such that minimal mechanical impedance exists in the plane of the diaphragm, and yet maintains the close spacing of the diaphragm to the perforated member. The support ring is formed in the perforated member and sets the size of the active part of the diaphragm. The height of the support ring defines the initial spacing between the diaphragm and the perforated member. There are one or more openings in the diaphragm and perforated member, providing an acoustical path from the back chamber of the transducer to the surroundings thereby eliminating any barometric pressure from building up across the diaphragm. The low roll-off frequency of the transducer is limited by the corner frequency formed by the acoustical resistance of said openings and the narrow gap between the diaphragm and substrate in combination with the acoustical compliance of the transducer back chamber. The perforated member has a systematic pattern of openings providing a low acoustical resistance of the air flowing to and from the air gap between the movable diaphragm and the perforated member. The systematic pattern and size of the openings are chosen such that the high roll-off frequency of the transducer is limited by the corner frequency introduced by the acoustical resistance in combination with the acoustical compliance of the diaphragm and back chamber of the transducer. This acoustical resistance is largely responsible for the acoustic noise generated in the device. As will be appreciated by those having skill in the art, there is a tradeoff to be made between damping and noise.
The perforated member, support ring, suspension means, and diaphragm can be made from a silicon wafer using micro machining thin-film technology and photolithography and can be made of one or more materials from the group consisting of: carbon-based polymers, silicon, polycrystalline silicon, amorphous silicon, silicon dioxide, silicon nitride, silicon carbide, germanium, gallium arsenide, carbon, titanium, gold, iron, copper, chromium, tungsten, aluminum, platinum, palladium, nickel, tantalum and their alloys.
In another embodiment the present invention also features an acoustic transducer consisting of a perforated member and a movable diaphragm spaced from the perforated member. The spacing is maintained by a support ring attached to the perforated member upon which the diaphragm rests. There are means for suspending the diaphragm such that the diaphragm is free to move in its own plane, thereby maximizing the mechanical sensitivity of the diaphragm. The suspension is achieved by utilizing high compliance springs between the diaphragm and perforated member. The spring assists in the construction and diaphragm release process, but once in operation the electrostatic attraction brings the diaphragm into contact with the perforated member support structure. Contrary to that taught by U.S. Pat. No. 5,146,435 to Bernstein, and U.S. Pat. No. 5,452,268 to Bernstein, the spring of the present invention plays an insignificant role in establishing the diaphragm compliance. There also are means for applying an electrical field between the perforated member and the diaphragm. There are further means for detecting the change of electrical capacitance between the perforated member and the diaphragm as the diaphragm deflects due to an incident acoustic sound pressure wave.
The thickness and size of the diaphragm is chosen such that the resonance frequency of the diaphragm is larger than the maximum acoustical operating frequency. Similarly, the dimensions of the perforated member are chosen such that the resonance frequency is larger than the maximum acoustical operating frequency. The perimeter at which the perforated member is attached to the substrate can optionally be shaped to minimize the curvature of the perforated member due to intrinsic stress in said perforated member. The high compliance suspension springs are made rigid enough for the structure to be made by micro machining technology, and yet compliant enough to mechanically decouple the diaphragm from the perforated member and to ensure that the in-plane resonance frequency of the diaphragm and springs is as small as possible compared to the intended low roll-off frequency of the transducer to prevent in-plane vibration of the diaphragm in operation. The support ring is formed in the perforated member and sets the size of the active part of the diaphragm. The height of the support ring defines the initial spacing between the diaphragm and the perforated member. There are one or more openings in the support ring, providing an acoustical path from the back chamber of the transducer to the surroundings thereby eliminating any barometric pressure from building up across the diaphragm. The low roll-off frequency of the transducer is limited by the corner frequency formed by the acoustical resistance of the openings and the acoustical compliance of the back chamber. The perforated member has a systematic pattern of openings providing a low acoustical resistance of the air flowing to and from the air gap between the movable diaphragm and the perforated member. The systematic pattern and size of the openings are chosen such that the high roll-off frequency of the transducer is limited by the corner frequency introduced by the acoustical resistance in combination with the acoustical compliance of the diaphragm and back chamber of the transducer. The perforated member, support ring, suspension means, and diaphragm can be made from a silicon wafer using micro machining thin-film technology and photolithography and may be made of one or more materials from the group consisting of: carbon-based polymers, silicon, polycrystalline silicon, amorphous silicon, silicon dioxide, silicon nitride, silicon carbide, germanium, gallium arsenide, carbon, titanium, gold, iron, copper, chromium, tungsten, aluminum, platinum, palladium, nickel, tantalum and their alloys.